Ultrastructural localization of platelet-derived growth factor and related factors in normal and transformed cells

Two-color fluorescence labeling in acrolein-fixed brain tissue

Acrolein is a potent fixative that provides both excellent preservation of ultrastructural morphology and retention of antigenicity, thus it is frequently used for immunocytochemical detection of antigens at the electron microscopic level. However, acrolein is not commonly used for fluorescence microscopy because of concerns about possible autofluorescence and destruction of the luminosity of fluorescent dyes. Here we describe a simple protocol that allows fine visualization of two fluorescent markers in 40-mum sections from acrolein-perfused rat brain. Autofluorescence was removed by pretreatment with 1% sodium borohydride for 30 min, and subsequent incubation in a 50% ethanol solution containing 0.3% hydrogen peroxide enhanced fluorescence labeling. Thus, fluorescence labeling can be used for high-quality detection of markers in tissue perfused with acrolein. Furthermore, adjacent acrolein-fixed sections from a single experiment can be processed to produce high-quality results for electron microscopy or fluorescence labeling.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

Decreased immunoreactivity of platelet-derived growth factor B chain-like peptide after axotomy in the dorsal motor nucleus of the vagus nerve

Platelet-derived growth factor-B chain (PDGF-B) and B chain-specific beta receptor (PDGF-R) were investigated immunohistochemically in the dorsal motor nucleus of the vagus nerve and hypoglossal nucleus after axotomy using antibodies raised against synthetic polypeptides. PDGF-B and PDGF-R immunoreactivity were observed in nerve cell bodies contralateral to the axotomized nerve in both vagal (degenerative) and hypoglossal (regenerative) nuclei. The immunoreactivity for PDGF-B antibody persisted until day 28 after axotomy in the hypoglossal neurons, while that in many neurons in the vagal nucleus diminished after day 3. In the severed vagal nucleus some of the axotomized neurons showed no immunoreactivity for PDGF-B chain, and these changes preceded the decrease in neuronal numbers in the vagal nucleus. The immunoreactivity for PDGF-R antibody showed no marked change in either the vagal or hypoglossal nucleus until day 28 after axotomy. These findings suggest that the decrease in PDGF-B immunoreactivity is not due to a non-specific depletion of cytoplasmic protein in the severed vagal neurons. PDGF, taken up by the nucleus and bound to chromatin, has been reported to exert direct effects on the enhancement of transcription and synthesis of RNA. The decrease in level of PDGF-B chain in the vagal neurons seems to cause the reduction of RNA and protein synthesis, resulting in neuronal degeneration.

PDGF-receptor concentration is elevated in regenerative muscle fibers in dystrophin-deficient muscle

Dystrophin-deficient muscle undergoes sudden, postnatal onset of muscle necrosis that is either progressive, as in Duchenne muscular dystrophy, or successfully arrested and followed by regeneration, as in most muscles of mdx mice. The mechanisms regulating regeneration in mdx muscle are unknown, although the possibility that there is renewed expression of genes regulating embryonic muscle cell proliferation and differentiation may provide testable hypotheses. Here, we examine the possibility that necrotic and regenerating mdx muscles exhibit renewed or increased expression of PDGF-receptors. PDGF-binding to receptors on muscle has been shown previously to be associated with myogenic cell proliferation and delay of muscle differentiation. We find that PDGF-receptors are present in 4-week-old mdx mice in muscles that undergo brief, reversible necrosis (hindlimb muscles) or progressive necrosis (diaphragm), as well as in 4-week-old control mouse muscles. Immunoblots indicate that the concentrations of PDGF-receptors in 4-week-old dystrophic (necrotic) and control muscles are similar. Prenecrotic, dystrophic fibers and control fibers possess some cell surface labeling of fibers treated with anti-PDGF-receptor and viewed by indirect immunofluorescence. Necrotic fibers in dystrophic muscle show cytoplasmic labeling for PDGF-receptors and labeling of perinuclear regions at the muscle cell surface. Adult dystrophic muscle displays higher concentrations of PDGF-receptor in both regenerated muscle (hindlimb) and progressively necrotic muscle (diaphragm) than found in controls. Anti-PDGF-receptor labeling of regenerated, dystrophic muscle is observed primarily in granules surrounding central nuclei or surrounding nuclei located at the surface of regenerated fibers. No labeling of perinuclear regions of control muscle or prenecrotic fibers was observed. Myonuclei fractionated from adult mdx hindlimb muscles contained no PDGF-receptor, indicating that PDGF-receptor-positive structures are not tightly associated with nuclei or within nuclei. L6 myoblasts show PDGF-receptor distributed diffusely on the cell surface. Stimulation of L6 myoblasts with 10 ng/ml of PDGF-BB causes receptor internalization and concentration in granules at perinuclear regions. Thus, PDGF stimulation of myoblasts causes a redistribution of PDGF-receptors to resemble receptor localization observed during muscle regeneration. These findings implicate PDGF-mediated mechanisms in regeneration of dystrophic muscle.

Immunogold labelling in combination with cryoultramicrotomy, freeze-etching, and label-fracture

During the past years, the methods of ultrastructural visualization of intracellular and cell-surface proteins have been improved considerably, mainly as the result of the development of low-temperature preservation in combination with immunocytochemical labelling procedures using poly- or monoclonal antibodies. In this contribution we will discuss the combination of immunogold labelling with cryoultramicrotomy and two replica methods, i.e. freeze-etching and label-fracture. The main advantage of cryoultramicrotomy is that it enables post-sectioning labelling, thus providing complete accessibility of all cellular antigens, located both intracellularly and on the cell surface. Important parameters that influence the labelling (i.e. label-efficiency), including penetration of the label and antibodies in the section, effects of fixatives on antigenicity, and steric hindrance, will be discussed in detail. The replica methods have the advantage of enabling an analysis of the lateral distribution of antigens located at the cell surface. The label efficiency is of particular importance in these studies and in this context several parameters will be discussed, including accessibility and effect of fixatives.

Growth-regulatory factors for normal, premalignant, and malignant human cells in vitro

Normal human cells, cells from nonmalignant proliferative lesions, and primary and metastatic tumor cells can be maintained in vitro and analyzed for requirements for growth in chemically defined media. The human melanocytic cell system with normal melanocytes, precursor nevus cells, and primary and metastatic melanoma cells has been extensively studied for the phenotypic properties of the cells, including their requirements for exogenous growth factors and other mitogens. In high calcium-containing W489 medium, normal melanocytes require four supplements: IGF-I (or insulin); bFGF, TPA, and alpha-MSH. Nevus cells are largely independent of bFGF. Depletion of TPA from medium is not as detrimental to nevus cells as it is to melanocytes, but the phorbol ester is still essential for maintenance of the typical nevic phenotype. Primary melanoma cells require at least one growth factor, IGF-I (or insulin), for continuous proliferation. On the other hand, metastatic cells of melanoma as well as of carcinomas of colon and rectum, bladder, ovary, and cervix are able to proliferate after a short adaptation period in medium depleted of any growth factors and other proteins. Doubling times of metastatic tumor cells in protein-free medium are only 30-60% longer than in FCS-containing medium. The growth autonomy of human tumor cells is apparently due to the endogenous production of growth factors. Likely candidates for autocrine growth stimulation of human tumor cells are TGF-alpha, TGF-beta, and PDGF. Melanoma and colorectal carcinoma cells express functional EGF/TGF-alpha receptors, and produce TGF-alpha, indicating that this growth factor is produced for autocrine stimulation. In addition to the use of anti-growth factor antibodies, other strategies for the inhibition of autocrine growth stimulation include mAbs to growth factor receptors, soluble receptors, receptor-mimicking antiidiotype antibodies, and active immunization against growth factors. Whether any of these therapeutic approaches is clinically feasible will need to be determined in extensive preclinical investigations.

Antagonistic effect of PDGF and NGF on transcription of ribosomal DNA and tumor cell proliferation

The molecular mechanism by which NGF and PDGF affect growth of tumor cells was tested in human melanoma WM 266-4 and colorectal carcinoma SW 707 cell lines. We present evidence that NGF translocated to the nucleus and bound to the chromatin of SW 707 cells, which express the cell surface and the chromatin receptor for NGF, inhibits ribosomal RNA synthesis which in consequence leads to inhibition of cell proliferation. In WM 266-4 cells, which do not express NGF receptor, NGF does not affect cell proliferation. In contrast, PDGF translocated to the nucleus of both SW 707 and WM 266-4 cells activates ribosomal RNA synthesis. We report here that NGF abolishes PDGF-activated ribosomal RNA synthesis and PDGF-stimulated growth of tumor cells.